Monopulse radio-detection receivers



United States Patent i 3,181,160 MGNOPULSE RADIO-DETECTION RECEIVERS Serge Pichafroy, Paris, France, assiglor to Compagnie Francaise Thomson-Houston, Paris, France, a French body corporate Filed May 22, 1961, Ser. No. 111,754 4 Claims. (Cl. 343-113) The present invention relates to a receiver which may be used in radio-detection systems known as monopulse systems.

The receiver of such a system measures the angular separation in elevation and in bearing between the direction of the object and the direction in which the aerial is pointing. To this end a receiving aerial is constructed with several elements receiving the echo pulse sent back by the object. From the amplitudes of the signals received in these elements is determined the value of the angular separation.

It is known to construct aerial structures with four elements arranged at the angles of a square perpendicular to the axis of pointing and having either one horizontal diagonal or two horizontal sides.

Such a system first of all effects the comparison of the difference in elevation AS and the difference in bearing AG signals, obtained from the signals received following the same diagonal or the same side, with the reference signal E obtained by calculating the sum of the signals received by all the'receiver elements.

In order to make this comparison, a three channel receiver can be used in each of which channels a transposition in intermediate frequencies and amplification of the signals AG, AS and 2 are effected.

An automatic control system operates on the gains of the three amplifiers in such a way as to render them equal to K/E, K being a constant and 2 the total amplitude of the signal. Y

Thus at the output of the three paths are obtained three pulse modulated voltages of amplitude KAG KAS 2 a and K2 -E- K By multiplying the two first by the last, two pulse modulated voltages of amplitude KZAG and

KZAS

the instabilities of the very low frequency electronic devices.

The receiver, which is the object of the present invention, does not have the aforementioned defects. It can be associated with a radio-detection system comprising one of the two types of aerials which have already been discussed. In this receiver, two voltages, the amplitudes of which are respectively proportional to n 2 and ab: E

are alternating and have the same frequency. This latter is different from the intermediate frequency common t0 the two channels and from the frequencies of the parasitic components which can appear in the course of the frequency-changing or multiplication operations.

In a receiver according to the invention the three channels (the two difference channels and the sum channel) have, as is usual, a frequency converter with which is associated an oscillator of frequency f1 but each of the difference channels, or preferably the sum channel, comprises a second converter with which is associated a local oscillator of frequency f2 which is different from the first local frequency and from the iirst intermediate frequency F-f1, F being the frequency of the echoes received. The voltage coming from the sum channel on the one hand and each of the voltages coming from the difference channels on the other hand are then mixed. The cornponents having the frequency of the local oscillator associated with the second converter (or converters) ofone of the types of channel are selected from the voltages produced by filtering. This is a pulse component and its amplitude is proportional to the product of the sum signal amplitude and the amplitude of one of the difference signals.

Each alternating voltage of frequency f2 modulated by pulses is added to a sinusoidal Voltage of the same frequency and, after the appropriate transformations, the resultant voltages are used to control the aerials directing servo-mechanisms. The variations in amplitude of each of the voltages thus obtained and the directions of these variations are respectively a function of the two vsum-difference products EAG, and AES. According to the method of using these voltages produced by addition, it is desirable that their amplitude varies as the sumdiferencerproducts which they represent or, on the other hand, that these amplitudes vary in opposite directions to these products. In order that it may be so, each operation of addition is carried out after having regulated the pliers have two components which are modulated in amplitude. The carrier Vfrequency of the one is double the intermediate frequency and it must be eliminated. The other has a veryweak carrier frequency, which is substantially zero. This component is extracted with the aid of a low pass filter. It has two sub-components. One of them is constituted by the video frequency signals in a usable form, and the other is a Very -low frequency parasitic sub-component arising from the asymmetry of the elements of the multiplier device. It should be noted that this latter sub-component undergoes a drift due to phase shift of one of the voltages, to which it is applied, in relation to the other.

The alternating voltages obtained by' addition must undergo a double transformation before being applied to the servo-mechanisms in order to` control them. These alternating voltages are first of al1 detected and then the resultant pulse voltages are transformed into very wide square Waves voltages by known devices called box cars.

In orderV that the technical characteristics of the present invention may be better understood, an embodiment will be described, by wayV of example with reference to the accompanying drawings, in which the single figure is a block diagram of a monopulse radio-detection receiver according to the invention.

The radio-detection system in the figure comprises an aerial 1, which is connected to the duplexer 2 and to the receiver 5 by the differential coupler 3, the function of which will be explained later. Upon transmission the Y duplexer 2 directs the energy coming from the transmitter 3 d towards the coupler 3, upon reception the energy received by the aerial and transmitted by this coupler 3 is fed to the three channels 6, '7 and S of the receiver, the duplexer 2 making sure that the energy which should reach the channel 7 is not directed towards the transmitter.

The aerial is essentially constituted by four radiating and receiving elements a, ,6, fy, and which are arranged at the angles of a square, the plane of which is perpendicular to the pointing axis, two sides of the square being horizontal.

Upon transmission the differential coupler carries out the equal distribution of the energy produced by the transmitter between the four radiating elements of the aerial. Upon reception the latter respectively receive signals, a, b, c and d, which are generally different and which are applied to the four inputs of the differential coupler 3. The latter combines the signals which are applied to it in a linear manner and produces A sum signal E=a+b+c+d An elevation-difference signal AS=(a-l-b)-(c+d) And a caring-difference signal AG: (a-l-d)-(b+c) These signals are respectively applied to the outputs indicated by the symbols E, AS and AG.

in each channel of the receiver the signals with a high frequency F undergo a transposition of frequency. To this end a local oscillator 9 of frequency f1 is connected by means of the guide 1? and the directive couplers 11, l2 and 13 to the sum channel and to the two difference channels respectively. In each of said directive couplers lll, i2 and f3, the signals are mixed with the local oscillator signal, the outputs thereof being fed to the respective amplifiers 14, l and 16 with an intermediate frequency F-f1- Each directive coupler is also provided with a terminating load. An automatic gain control device 17 fed from the amplifier f4 is associated with all three amplifiers and has the effect of making the amplitude of the signal produced by the amplifiers constant. A second frequency converter i3 forms part of the sum channel and a local oscillator 22 of frequency f2 different from f1 is associated with it. A pass-band filter 19 selects the useful component of the voltage produced by the frequency converter; this filter has a mean frequency of, for example, F-fl-l-fz. It should be noted that, if desired, the component of frequency F-fl-fz of the voltage produced by the mixer 18 may be used. The mean frequency of the filter l@ should then therefore be modified.

The voltage transmitted by this filter is applied to two identical operative devices Ztl and 2l, each of which has two functions. The device 2@ serves to effect the product of the sum signals transmitted by the filter 19 and the difference-in-elevation signals. Furthermore this device carries out the addition of the pulse modulated alternating voltage which represents this product and a voltage of frequency f2, the resultant voltage being used, after suitable transformation, to control the servomechanisms for pointing in elevation. The device 2l is identical to the device Ztl but it operates on the sum voltage and the bearing difference voltage. It produces a voltage which is also used after transformation to control the bearing pointing servo-mechanisms. Since the devices 20 and 2l are identical only one of them will be described. Similar members of the two devices are indicated by the same reference numerals but those of the device 2l are distinguished by a prime.

The filter i9 is connected to the first input of the mixer 23 which forms part of the device Ztl. The amplifier 15 of the elevation-difference channel is connected to the second input of the mixer 23 either directly or preferably through a phase shifter 24, which has the task of rendering the transmission times through the sum channel and the elevation-difference channel equal. That component is selected from among the components of the voltage produced by the mixer 23 which has a frequency f2 equal to the difference between the second intermediate frequency Ff1|f2 of the signals transmitted by the sum channel and the intermediate frequency F-f1 of the signals transmitted by the elevation-difference channel. This selection is carried out by the pass-band filter 25, the mean frequency of which is that of the local oscillator 2.2. The selected component of frequency f2 has an amplitude proportional to the product 2.AS of the amplitudes of the signals transmitted by the sum and elevation-difference channels.

This pulse modulated voltage of frequency f2 is -added to the voltage coming from the local oscillator 22 in the device 26. In order to increase the variations of amplitude of the voltage produced by the adder as much as possible, the voltage of the local oscillator 22 is transmitted to it preferably by a phase shifter 27. The latter is regulated so that the relative phasing of the voltages applied to the inputs of the adder be zero or equal to 1r. Two regulations of the phase shifter 27 can thus be effected for a given value of the angular error signal, whether the amplitude of the voltage produced by the adder increases during the echo pulses or decreases.

The voltage produced by the adder is applied to a detector 28; a pulse voltage is then obtained which is transformed into voltages with wide square waves (almost direct voltage) by a known device 29 called a box car. The elevation pointing servo-mechanisms may be controlled by the voltage which appears at the output of the device 29. It should be noted that the sum channel can comprise only one single frequency converter, whereas the two difference channels have two. The second converter of each of' these channels is then followed by a filter which is connected to the mixer of the device 2f) and 2l. In this case the amplifier 14 of the sum channel is followed by a phase shifter, the output of which is connected to the second input of the mixers Z3 and 23.

The voltage appearing at the output of the adder 26 has an alternating component of const-ant amplitude during the intervals between the pulses. it is important that the amplitude between the pulses of the voltage is not small in relation to the amplitude of these pulses for in this condition the distribution of the amplitudes of the noise component which obeys Rayleighs law is then close to a Gauss distribution. Under these conditions the voltage produced by the adder 26 is practically zero when there is no angular error of the antenna.

It should be noted that in the devices 20 and 21 and the adders 26 and 26 can be replaced by another device, a multiplier for example. The use of such a multiplier' has the advantage of allowing the more efiicient elimination of the parasitic components of the voltage produced by this device.

With the receiver 5, one may alternatively use an aerial, the elements of which are arranged at the angles of the square having a horizontal diagonal perpendicular to the pointing axis. The only dirlierence relates to the coupling device which connects the elements of the aerial and the three reception channels.

I claim:

1. Receiver for a monopulse radio detection system of the type which makes use of variations in the amplitude of the difference in elevation signal AS and of the difference in bearing signals AG in relation to the sum amplitude E of the signals and comprising two difference channels to which said difference signals are fed and a sum channel to which the sum amplitude signal is fed, frequency converter means fed with the signals from the two difference channels and the sum channel, a local oscillator producing a frequency f1 which is fed to the frequency converter means to produce an intermediate frequency, an intermediate frequency amplifier for each of said channels connected to the output of the frequency converter means, a second frequency converter means associated with the sum channel, a second local oscillator for feeding the second frequency converter means and from the output of each of themixer circuits.

2. Receiver as claimed in claim 1, in which the output from each mixer circuit is applied to an adder circuit which is also fed with the local oscillator frequency f2, a detector fed with the output of the adder circuit for producing a pulse voltage and a box car circuit fed from the detector for producing a square wave output 3. Receiver as claimed in claim 2, in which the local oscillator frequency f2 is applied to the adder circuit through a phase shifter such that the alternating voltage from Vthe adder varies in amplitude according to whether or not the sum voltage E and the respective difference voltages AS, AG are in phase.

4. Receiver fora monopulse radio detection system of the type which makes use of the variations in the amplitude of the difference in elevation signals AS and of the difference in bearing signals AG in relationto the sum amplitude 2 of the signals and comprising two difference channels to which said difference signals are fed and a sum channel to which the sum amplitude signal is fed,

frequency converter means fed with the signals from nal and the respective difference signals (2.AS,2 .AG)V

the two difference channels and the sum channel, a local oscillator producing a frequency f1 which is fed to the frequency converter means to produce an intermediate frequency, an intermediate frequency amplifier for each of said channels connected to the output of the frequency converter means, second frequency converter means associated with at least one of the channels, a second local oscillator for feeding the second frequency converter means and which produces la frequency f2 different from the rst local oscillator frequency f1 and from the intermediate frequency, two mixer circuits connected to the at least one channel which is not connected to the second frequency converter circuit and which respectively multiply the voltage 2 with the voltages AS and AG and means for selecting the alternating component of frequency f2 and of an amplitude proportional to the product of the sum signal and of the respective difference signals (E.AS,E.AG) from the output of each of the mixer circuits.

References Cited bythe Examiner UNITED STATES PATENTS CHESTER L. JUSTUS, Primary Examiner. 

1. RECEIVER FOR A MONOPULSE RADIO DETECTION SYSTEM OF THE TYPE WHICH MAKES USE VARIATIONS IN THE AMPLITUDE OF THE DIFFERENCE IN ELEVATION SIGNALS $S AND OF THE DIFFERERENCE IN BEARING SIGNALS $G IN RELATION OF THE SUM AMPLITUDE $ OF THE SIGNALS AND COMPRISING TWO DIFFERENCE CHANNELS TO WHICH SAID DIFFERENCE SIGNALS ARE FED AND A SUM CHANNELS TO WHICH THE SUM AMPLITUDE SIGNAL IS FED, FREQUENCY CONVERTER MEANS FED WITH THE SIGNALS FROM THE TWO DIFFERENCE CHANNELS AND THE SUM CHANNEL, A LOCAL OSCILLATOR PRODUCING A FREQUENCY F1 WHICH IS FED TO THE FREQUENCY CONVERTER MEANS TO PRODUCE AN INTERMEDIATE FREQUENCY, AN INTERMEDIATE FREQUENCY AMPLIFIER FOR EACH OF SAID CHANNELS CONNECTED TO THE OUTPUT OF THE FREQUENCY CONVERTER MEANS, A SECOND FREQUENCY CONVERTER MEANS ASSOCIATED WITH THE SUM CHANNEL, A SEOCOND LOCAL OSCILLATOR FOR FEEDING THE SECOND FREQUENCY CONVERTER MEANS AND WHICH PRODUCES A FREQUENCY F2 DIFFERENT FROM THE FIRST LOCAL OSCILLATOR FREQUENCY F1 AND FROM THE INTERMEDIATE FREQUENCY, TWO MIXER CIRCUITS CONNECTED IN THE TWO DIFFERENCE CHANNELS AND WHICH RESPECTIVELY MULTIPLY THE VOLTAGE $, WITH THE VOLTAGES $S AND $G, AND MEANS FOR SELECTING THE ALTERNATING COMPONENT OF FREQUENCY F2 AND OF AMPLITUDE PROPORTIONAL TO THE PRODUCT OF THE SUM SIGNAL AND THE RESPECTIVE DIFFERENCE SIGNALS ($.$S,$.$G) FROM THE OUTPUT OF EACH OF THE MIXER CIRCUITS. 